The present invention relates to apparatus for modulating the speed of a traveling web. More particularly the invention relates to apparatus for enabling a workstation to operate on a traveling web for an increased period of time by temporarily decreasing the velocity of the section of web as it passes through the workstation or operational zone where it is acted upon as a part of the processing of the web.
An example of a process wherein the invention is applicable is in the production and manufacture of disposable products such as pants-type diapers or protective undergarments. It is often necessary that bonds be created transverse to the machine-direction of a web used to form such products. These bonds might, for instance, be required to effect the side-seal on protective undergarments. Additionally, these products might require that a number of layers of materials be bonded securely, creating a bond with a high mechanical strength.
The types of nonwoven fabrics normally used in disposable articles may have inherent thermal bonding characteristics, but multiple layers of this material tend to act as thermal insulators, inhibiting the rapid penetration of heat to the innermost layers. This insulating quality may limit the ability of simple heat and pressure from producing effective bonds at reasonable production speeds. Typical protective undergarment assemblies may have areas of four, six or even eight layers of material to be bonded.
One method of thermo-mechanical bonding commonly employed involves the application of ultrasonic energy to the web between an ultrasonic actuator horn which engages the web assembly while it is supported on its opposite side by a patterned anvil. This method applies bonding energy to all layers of the web simultaneously in what is effectively a hammer-forging process. A series of very rapid, very intense hammer blows results in a very localized form of heat and pressure, transmitted to every layer of the material. When enough hammer blows are applied with sufficient intensity, the materials fuse into a well integrated laminate.
A common method of applying such ultrasonic energy to a moving web is to pass the web between a vibrating, but otherwise stationary ultrasonic horn and a drum-shaped rotating anvil which is provided with a plurality of patterned tools around its periphery. The tools are spaced apart according to the product pitch (i.e., length of individual product web segments) in the machine direction.
It can be appreciated that each configuration of material, numbers of layers of material and bonding pattern employed in various products may require a minimum number of ultrasonic hammer blows to create an acceptable bond. Further, it can be appreciated that the speed with which the web passes through the bonding zone between hammer and anvil limits the number of blows that can be applied at a given hammer frequency. Given the dimensions of a particular web, a maximum attainable ultrasonic hammer vibrational frequency and a limited effective hammer width, it can be seen that there will be a maximum web speed at which effective bonds can be reliably produced. In view of these considerations, it will be appreciated that a need has existed for processes and apparatus for increasing the effective speed of such procedures.
The invention provides processes and apparatus designed to increase the speed at which a web can be processed through an intermittent work station such as an ultrasonic bonding device. The apparatus serves to modulate the velocity of the web segment worked on at the bonding station, slowing it during that part of the cycle during which bonds are to be produced, so that the bonds may be effected at a velocity lower than the nominal web velocity. Festooning systems are commonly used to either stop or reduce the velocity of a moving web at a selected location in a process. This invention provides the benefits of festooning without the disadvantages normally associated with linear motion mechanisms.
In accordance with a preferred embodiment of this invention, the speed controlling effect is produced by moving two web transporting rollers located on opposite sides of the bonding device, ie., upstream from and downstream from the bonding device, in concert with one another and in a line of motion parallel to the machine direction of the traveling web. In this embodiment, each roller is configured to produce a linear displacement by rotating an outer cylinder with an internal eccentric bore about a shaft which is provided with an equally eccentric outer surface. The shaft is rotated synchronously to and in the opposite direction of the outer cylinder. Inasmuch as the amounts of eccentricity are equal and due to the fact that they are rotating in opposite directions, the additive displacements are such that motion appears to be linear and applied in a single radial direction.
Briefly summarized, according to one important aspect, the invention provides process and apparatus which modifies the speed of a travelling web so that it can have an operation performed on it at a workstation at a desirably reduced velocity while maintaining a high overall web velocity. An important such operation consists of ultrasonically bonding the layers of a web which contains thermoplastic materials such as non-woven fibers. First and second process roller assemblies located at the entry and exit points of an ultrasonic bonding device are simultaneously and cycloidally shifted along a line tangential to the bonding point. The first and second process roller assemblies are each equipped with eccentric mechanisms, which are in turn mounted on second supporting eccentric mechanisms such as shafts or hollow cylinders, which are rotated about equally eccentric centers. The process rollers of the first and second roller assemblies are rotated in the direction of the web passing over them, while the first and second supporting mechanisms are rotated in directions opposite those of their respective rollers. Each roller and each supporting eccentric mechanism is synchronously rotated, so that each makes one revolution per product length. As each eccentric mechanism rotates relative to its respective counter-rotating roller, the directional displacements of the eccentricities are canceling in one axis, while they are additive in another, perpendicular axis. This arrangement imparts a festooning motion to the web, wherein its velocity varies cycloidally by an amount dependent on the amount of eccentric displacement. This festooning effect is, thus, achieved by simultaneously shifting the first and second roller assemblies in a direction counter to the machine direction each time a bonding force is applied or other operation performed. The overall running speed in the machine remains at a constant velocity while the dwell time for bonding operation is increased.
In a preferred embodiment the first and second rollers of each roller assembly are eccentrically rotationally mounted, respectively, on third and fourth hollow rollers which are provided with eccentrically positioned hollow cylindrical openings which, in turn, are mounted, respectively, on shafts positioned perpendicularly to said machine direction. Shifting of the rollers is effected by simultaneous rotation of the eccentric mechanisms.
In accordance with further aspects of the invention, the same process and apparatus is applicable to various other operations conducted with respect to a traveling web at a work station. Such operations can include mechanical bonding, thermal bonding, crimping, stamping, embossing, printing, spraying, heating, irradiation, cutting, sewing, application of additional layers or appendages, or a myriad of similar operations, as will be appreciated by those skilled in the art.
As used herein, xe2x80x9cthermo-mechanical bondingxe2x80x9d refers to ultrasonic bonding procedures and to other processes that use a combination of heat and pressure to unify or bond together a plurality of layers of a web of bondable material. Ultrasonic horn devices are intended to be excited at frequencies ranging from about 18 to about 60 kHz. Such devices are known in the art and do not form a direct part of this invention. Details of such devices are set forth, for example, in U.S. Pat. No. 5,096,532 to Neuwirth et al, issued Mar. 17, 1992 and in the references cited in said patent disclosure. See also U.S. Pat. No. 5,421,924 issued Jun. 6, 1995 to Ziegelhoffer et al.